The human macrophage line U927 and the human T-lymphoblastoid line HSB2 have been rendered sensitive to hypoxanthine-aminopterin-thymidine (HAT) culture medium by treatment with 8-azaguanine. The HSB2 cell line was fused to T lymphocytes that were stimulated with concanavalin A for 48 hours. Hybridomas were generated that constitutively produced interleukin 2 and chemotactic factor. Eight of these hybridomas have been cloned and continue to constitutively secrete (greater than 12 months) interleukin 2 in concentrations from 2 to 40 times that of an equal number of mitogen-stimulated peripheral blood lymphocytes. In another experiment, HSB2 was fused to human T lymphocytes stimulated with concanavalin A for 24 hours. Three T-cell hybridomas were generated and cloned that constitutively secreted macrophage activating factor. Fusion of the U937 line with purified human monocytes has recently generated six human monocyte hybridomas that have been successfully cloned. The characteristics of these hybridomas and their secretory products are currently being investigated.